1. Field of Invention
The present invention relates generally to machines adapted to test the wear, wear-preventative and friction properties of oils, grease, dry-film lubricants and other lubricants, and both lubricated and non-lubricated materials.
More particularly, the invention relates to machines of a type adapted to test such properties between a rotating test specimen loaded against one or more non-rotated, generally stationary test specimens in a direction along the axis of rotation of the rotating specimen.
2. Description of Prior Art
A common conventional test machine of the subject type typically used for testing the wear preventative characteristics of lubricants is known as a four-ball test machine. In particular, four-ball test machines are generally used to conduct two basic types of testing: a wear test, and extreme pressure tests.
During a typical wear test in a four-ball test machine, one ball is rotated against three non-rotated, generally stationary balls under predetermined time and load conditions. The balls are coated with or immersed in a test lubricant during the test, and the wear patterns on the balls are analyzed after the test to determine the wear-preventive characteristics of the lubricant.
Standardized test methods of this general type are explained more fully in ASTM D 2266, Standard Test Method for Wear Preventive Characteristics of Lubricating Grease (Four-Ball Method); and ASTM D4172, Standard Test Method for Wear Preventive Characteristics of Lubricating Fluids (Four-Ball Method).
Common extreme pressure tests, conducted under relatively high loads, include: a weld-point test to determine at what test load the balls seize-up or weld together with the lubricant being tested, and a load-wear index test to determine an index of the ability of the lubricant to prevent wear at an applied load.
Standardized test methods for tests of this general type are explained more fully in ASTM 2596, Standard Test Method for Measurement of Extreme-Pressure Properties of Lubricating Grease; and 2783, Standard Test Method of Extreme-Pressure Properties of Lubricating Fluids (Four-Ball Method).
A conventional four-ball test machine includes a ball chuck to hold the ball to be rotated, and a ball pot in which the three stationary balls are held and which is sized to immerse the test balls when testing a liquid lubricant. The stationary balls are held centered about the axis of rotation of the ball chuck, and the rotated ball is loaded against the three stationary balls with a test force or load acting along the axis.
In a lever-loaded machine, the test load is applied to the balls with the use of weight at one end portion of a lever-arm. The lever arm is fulcrumed in the machine such that the other end engages a linear acting rod or pin which in turn axially loads the balls through a thrust bearing and thrust plate.
In setting up for a test, the operator manually positions a weight to act over a lever-arm length to obtain the desired load condition on the balls. The test load is adjusted by adjusting the weight and its position along the length of the lever, and is determined by the weight and a standard lever formula associating the ratio of the distances from the fulcrum to the weight and to the load rod.
There are several disadvantages and drawbacks associated with the use of conventional, lever-loaded wear test machines. In particular, it is well known that the test results obtained with a conventional lever-loaded four-ball test machine are not accurately repeatable. This is particularly true as between different test setups. See e.g., discussion of repeatability of test results in ASTM D 2266.
The inability to accurately reproduce test results with conventional four-ball test machines is due to several factors, including:
(i) Non-Reproducibility of Test Load Conditions—An operator is often unable to accurately reproduce load conditions between different test setups, whether on different machines or the same machine because, among other things, of possible load error and positioning error in manually selecting and positioning the load on the arm, and because the machines do not provide confirmation of the load condition established by the operator prior to conducting a test.
(ii) ASTM accepted accuracy for an applied load is only approximately +0.5% (e.g., +0.2 kg at 40 kg load)
(iii) Operator Dependency—Test results are also dependent upon the operator conducting the test. See e.g., ASTM D 2266, paragraphs 11.2.1 and 11.2.2 in which anticipated differences between test results are shown to be higher with different operators than with the same operator, using the same test setup.
(iv) Friction Induced Error—The rod that transfers the load from the lever to the test balls is subject to a stick-slip friction condition, resulting from side-loading of the rod against its guide bore and other components such as contacting the ends of the rod. In addition to other effects, and as discussed further below, this friction makes it difficult to obtain the same test load from one test setup to another, and maintain a constant load during the test as the specimen wears. As a result, the stick-slip friction problem lowers the sensitivity, accuracy and repeatability of the test machine.
Linear wear between the test specimens during a test is another indicator of the characteristics of the lubricant under test. However, the measurement of linear wear with conventional lever-loaded wear test machines is not accurately repeatable between tests.
Measurement of linear wear in a conventional test machine is accomplished indirectly with an LVDT that extends parallel to but is offset from the axis of the applied load. The LVDT is connected between a stationary frame member of the machine, and the outer end of a small lever that engages the ball pot assembly at the other end for linear movement therewith, and that extends radially outwardly therefrom to the LVDT.
However, variable and unpredictable vibration occurring during a test, and amplification of the vibration along the length of the small lever arm, introduces errors into the setup, which results in LVDT sensing and indication of linear wear that is not the true wear between the test specimens. Thus, use of such arrangements does not provide linear wear data with a high degree or accuracy, but only provides a relatively rough indication of linear wear during a test.
In addition, the load range of a conventional lever-loaded four-ball wear test machines is limited, being typically rated for a maximum test load of approximately 60 kg. This load range is insufficient to weld most materials, and thus, the conventional four-ball test machines are not capable of conducting the extreme pressure test that are also indicative of lubrication and wear characteristics. Therefore, if both conventional wear tests and extreme pressure tests are to be conducted, a second test machine capable of conducting the extreme pressure tests is required.
The friction induced test deficiencies are due, in part, to relatively small clearances in the lever-loaded wear test machine. In particular, the clearance between the load pin and the bore in which it is guided is relatively small. As a result of this small clearance, any side-loading on the pin results in friction between the pin and the bore. Side-loading also results in friction between the ends of the pin and its supporting components, including a thrust disk that transmits the load on through to the balls at one end of the lever.
In practice, the side-loaded pin exhibits a condition that is commonly known as a “stick-slip” condition. This condition occurs with a relatively high static friction coefficient as compared with the dynamic friction coefficient. As a consequence, the static friction must be overcome before the pin will begin to move. In other words, as a result of the stick-slip friction condition, a relatively high change of force must occur before the rod can move to maintain a constant load condition as the specimen wears. This is a condition that can cause problems both before and during a test.
Before the test, as the load is adjusted prior to a test, the full change in load may not reach the test balls if it is insufficient to overcome the static friction; and even when sufficient to overcome the static friction, a portion of the load change may be lost as reacted against by the static friction. In other words, loading of the lever with the weights exhibits a hysteresis band between the change in load from the lever and the load carried through to the balls. Consequently, it can be difficult to obtain the same test load from one test setup to another, even if the same weight is placed in precisely the same position on the lever.
And the balls wear during a test. This wear causes two direct consequences: (i) the load mechanism must move to maintain a constant load on the balls, and (ii) the surface of the balls becomes rough. In order to maintain a constant load on the test balls, the pin must move toward the balls. However, the stick-slip condition can prevent the pin from moving smoothly during a test to maintain a constant load.
The balls do not wear evenly during a test. This uneven wear can result in further side-loading of the pin against the guide bore. This side-loading is unpredictable and can change throughout the test, resulting in a changing test load during the test and further unrepeatability of test results.
This stick-slip condition reduces the sensitivity of the machine to changes in load, and thus reduces the ability to accurately control the load both before and during the test. Thus, it is apparent that conventional lever-loaded four-ball wear test machines are incapable of providing accurately repeatable test data, but rather provide only a relative indication of such data within relatively wide limits.
In addition to the conventional wear and extreme pressure tests, accurate real-time data as to linear wear of the test balls would be instructive as to and assist in determining additional parameters of the wear, wear-preventative and friction characteristics of the lubricant or material under test. However, conventional four-ball test machines are incapable of providing such data.
An alternate four-ball test machine uses a pneumatically actuated piston that is slidably located in a guide bore to load the test balls. In an effort to keep the friction low, this pneumatic operator does not use a piston seal, but instead relies on a relatively snug fit between the piston and the bore. However, this type of load mechanism is also subject to stick-slip friction as the piston attempts to slide during a test, resulting from side loading against the bore as the balls wear. In addition, the load range of this alternate test machine is typically limited to approximately 180 kg, and is thus inadequate to conduct the extreme pressure tests. This machine is also incapable of providing direct wear data with a high degree of accuracy during a test.
Extreme Pressure (EP) four-ball test machines are typically rated for maximum load conditions of approximately 1000 kg and are used to conduct the extreme pressure tests.
The construction of an EP lever-loaded four-ball test machine is similar to the conventional lever-loaded four-ball test machines, except that the lever and associated actuating and loading components in the load path are constructed for the higher loads.
However, conventional lever-loaded EP test machines also present several disadvantages and drawbacks. Although capable of conducting the weld-point test, it can be an extremely inefficient, time consuming and costly test as conducted on an EP machine.
The procedure for conducting a weld-point test on an EP machine involves first establishing a predetermined load, and running the balls under that load against the rotating ball for a predetermined time such as ten seconds.
If the test balls did not seize-up during the ten second run, the machine is turned off, new balls and test lubricant are put into the test machine, an increased load is established by adjusting the weight and/or the location of the weight along the lever arm, and the test is run for another ten seconds.
This procedure is repeated, with incrementally increasing loads, until the test balls seize during the ten second run; the load at which the balls seize being deemed the weld-point threshold of the lubricant under test. Without repeating the testing by reducing the load conditions in smaller increments, the test provides only a minimum threshold that the lubricant passed; it does not establish the actual load at which the balls would seize together. And determining the load-wear index is based on several applied loads immediately preceding the weld point. (See e.g., ASTM D 2596, Sect. 10.2 Load-Wear Index). Thus, determining the weld-point on an EP machine is a slow, repetitive, and expensive process that typically only establishes a minimum weld-point threshold, and conducting the load wear index test is based on the repetitive nature of the weld-point test.
The load accuracy and sensitivity of EP machines are even worse that the accuracy and sensitivity of the conventional wear test machines. The load accuracy of an EP machine is typically approximately +1 kg or greater. Therefore, EP machines are not suitable for accurately conducting the conventional wear testing under the lighter load conditions of conventional four-ball wear test machines. Thus, to conduct both low load wear testing and high load extreme pressure testing requires the use of two different machines, and the associated cost for purchasing both machines.
The accuracy and sensitivity of EP machines, and the repeatability of test results, are further reduced due to friction inherent in the design of the machine. As with the conventional four-ball wear test machines, the load pin experiences a stick-slip condition from side thrust imparted by the lever onto the pin against the side of the bore, and from uneven wear of the balls.
However, the higher load and wear conditions in an EP test machine enhance the effect of friction (i.e., the side-loading and resulting friction between the rod and the bore is enhanced), and thus the stick-slip condition is enhanced on EP machines.
As with the conventional wear test machines, EP test machines do not include provision for detecting with a high degree of accuracy the linear wear of the balls or torque during the test. However, of further concern in an EP machine, the lack of wear detection can result in damage to the machine. When the balls experience excessive wear, the skirt of the rotating chuck can contact the stationary balls. This results in damage to the chuck which must then be replaced, as well as loss of associated part and labor cost and downtime of the machine.
Damage to the chuck also presents the possibility of substantial damage to the machine when the chuck loses the ability to firmly hold the rotated ball. In which case, additional parts must be replaced, resulting in greater expenses and losses associated therewith.
EP machines are also subject to severe vibration problems, further reducing the accuracy and repeatability of the test results. As previously mentioned, the balls wear unevenly, and the surface finish of the balls becomes rough during a test. The rotation of balls having a rough surface finish against each other causes the machine components in the load path to vibrate. With the relatively high loads, this vibration level can become substantial during testing on an EP machine.
This high vibration is variable and unpredictable, and in many instances, is further amplified as the frequency of vibration approaches the natural frequency of the lever and load path components, and/or from the stick-slip friction phenomenon which can contribute to a dynamically unstable condition.
As a result, often times during EP machine extreme pressure testing, the load-path components bounce back and forth with relatively high amplitude. This high amplitude vibration, including bouncing of the weighted lever results in impact loading between the balls during the test, and further contaminates the test, complicates analysis of the test results, and reduces the repeatability of such results.
Yet another type of four-ball test machine, is a multi-specimen machine that uses either two or four pneumatic diaphragm operators to load the test balls. The pneumatic operators are located radially outwardly of the rotational axis of the rotated ball at an equal distance, are equally spaced from one another.
Although this type of machine offers certain potential improvements as compared with conventional lever-loaded wear and extreme pressure test machines, it also suffers from certain drawbacks and disadvantages that prevent it from fully implementing or realizing those potential improvements.
The multi-specimen test machine does not provide for direct measurement of the load or wear during a test. This machine senses the pressure in the pneumatic operators to determine the load on the balls. Therefore, control of the test load is subject to response errors, and the load information that is available is not accurate on a real-time dynamic basis, as it is subject to a lag, both resulting from the compressibility of the air in the pneumatic actuators.
In addition, the multi-specimen has a relatively high number of friction points, including two friction points on the load rod, and point of friction for each of the load cylinders. Thus results in a test machine with a relatively high hysteresis characteristic in the load system of the machine.
This multi-specimen test machine is typically rated for test load range of approximately 2,000 lb. Therefore, the load operators of the machine are capable of providing sufficient load to conduct the extreme pressure tests. However, as constructed, the load path components of the multi-specimen test machines preclude safely conducting the extreme pressure tests.
In particular, the structural characteristics of the spindle assembly of the multi-specimen machine are incapable of safely supporting the rotating ball against the high loads of the extreme pressure tests, because of the spindle design. The spindle has a long, relatively narrow portion between support bearings associated with the housing and the mounting end for the upper test specimen. As a result, the mounting end flexes and does not allow for table positioning of the upper test specimen, particularly under extreme-pressure load conditions.
From the foregoing discussion, it is clear that there is a need for a new and improved four-ball test machine that addresses the above-identified disadvantages and undesirable characteristics of conventional prior four-ball test machines.
In particular, there is a need for a machine that eliminates the stick-slip condition of prior test machines, and the friction induced problems associated therewith; eliminates or reduces the vibration induced problems associated with prior machines; provides for application of an accurate and repeatable test load condition, and for direct measurement of the test load condition in real-time for visual or other confirmation or control thereof; eliminates operator dependency in the application of the test load, and in the test results with visual feedback of actual load conditions to the operator, and/or with automatic control of the test load; provides improved load range capability to enable conducting both wear and extreme pressure tests on the same machine resulting in substantial cost savings over having to purchase and maintain two different machines; provides improved efficiency in conducting the extreme pressure tests, including accurately determining the actual weld-point threshold and improving accuracy of the load-wear index data; and provides direct measurement of wear and torque generated during a test, to preclude the possibility of damage to the machine due to excessive specimen wear, and to permit determination of additional test-result parameters and characteristics.